21 December 2010

The Purpose of Generic Names: Or, Everyone's a Homo

Vrſvs Lotor Linn.

It can be surprising for a modern-day biology student to look at 18th-century texts and see how broad the genera are. Consider Linnaeus: he named raccoons as a species of bear (Ursus lotor, the "washer bear"—still called "tvättbjörn" ["washbear"] in Swedish). Nowadays raccoons aren't even placed in the same family as bears, and bears are split into anywhere from roughly three to seven extant genera. Or consider bats, nowadays comprising about 60 extant genera, while Linnaeus classified them as just one (Vespertilio). These aren't even the craziest examples.

Was Linnaeus nuts? Of course not. He saw that the task of creating a unique name for every species would be extremely difficult, so he decided it would be okay to reuse the same names if they were prefaced by the name of a more general category—a genus. Thus, for example, he was able to call the house mouse Mus musculus and the blue whale Balaena musculus(now either Balaenoptera or Sibbaldus). Although they have the same specific epithet, those epithets are unique within their general category. As long as homonymy is avoided, there's no nomenclatural need to restrict genera, so why not make them broad?

In this way, genera function as what we in the computer science world call namespaces. Different things are allowed to have the same local name as long as they are within different namespaces. The qualified name, which combines a namespace identifier with the local name, is globally unique, even if the local name is not.

Biological Nomenclature

generic name + specific epithet = species name

Computer Science

namespace identifier + local name = qualified name

Recently, this got me to thinking—why have we restricted our genera so much, when this was their original purpose? If we just want to be sure that each species has a globally unique name, we could have much larger genera—in some cases, even larger than Linnaeus'.

Consider our own genus, Homo. It has exactly one extant species, Homo sapiens, and that species has an epithet that is already, as far as I know, globally unique. How is that useful? Even Linnaeus thought this was rather silly—he would have included humans in his anthropoid genus, Simia, except he feared backlash. (Even including us in the same order as other primates was controversial at the time.)

How far could we extend our genus and retain its usefulness as a namespace? How far out can we go without having duplicate local names? Within Homo we already have local names like sapiens, erectus, habilis, etc. We actually do have at least one duplicate name, Homo capensis, but it's universally considered a junior synonym. (Although this case is a bit complicated.) If we only consider valid, non-synonymous names, how far can we push our genus?

If we include all stem-humans there's no problem. We add things like Homo afarensis and Homo robustus.

Left to right: Homo sapiens, gorilla, troglodytes, lar, & pygmaeus.

If we push it out to the crown clade of African apes there's still no problem. We get things like Homo gorilla(western gorillas) and Homo troglodytes (common chimpanzees). (Admittedly Homo troglodytes was already named by Linnaeus, but it's a nomen oblitum without any specimens or certainty as to what, exactly, it was supposed to indicate.)

Still no problem if we push it out to the great ape crown clade, adding things like Homo pygmaeus(Bornean orangutans) and Homo indicus(otherwise Sivapithecus).

Pushing it out to the ape crown clade still works, as we add things like Homo lar(lar gibbons) and Homo syndactylus(siamangs). (Interestingly enough, Homo lar is Linnaeus' original name for the species. Only later was it given a new genus, Hylobates, by Illiger, where it resides to this day. I'm not quite sure why Linnaeus classified it this way, but my guess is he wasn't that familiar with the animal in question, as was often the case.)

But if we go beyond that, we hit a duplicate: Homo africanus (Hopwood 1933, originally Proconsul) and Homo africanus (Dart 1925, originally Australopithecus). (We actually already hit Meganthropus africanus Weinert, 1950 a while ago, but that's universally considered a synonym.)

So there we go, Homo could be used as the generic name for all crown-group apes without any problem. (I'm willing to bet I missed something, though, and I'm looking forward to some commenter correcting me.) We have restricted our genera far more than they need to be restricted in order for species names to be unique.

Are we nuts? Of course not. A genus is much more than just a namespace. We also use genera on their own, as groups in their own right. Expanding Homo to embrace all extant apes would ruin its utility as a name for a certain subclade of the human total group, and make it redundant with a name that we already have (Hominoidea). (In computer science namespaces are also often narrower than they technically could be.)

Then again, maybe it is a bit nuts to have one thing performing two jobs. Why not allow other, larger taxa to be used as namespaces? Well, under the PhyloCode, that will actually be a possibility. We can refer to Hominoidea syndactylus and Hominoidea sapiens if we want. In fact, I think you could use Synapsida syndactylus (avoiding homonymy with Bleda syndactylus, the red-tailed bristlebill) and Biota sapiens(this epithet being globally unique already, as previously mentioned). These particular examples will probably never be popular and I wouldn't use them myself, but I think it's neat that the possibility exists.

4 comments:

Recently, this got me to thinking—why have we restricted our genera so much, when this was their original purpose?

I don't think that's all there was to it, but I don't know if Linnaeus ever actually specified his 'genus concept'. Nor would he have necessarily felt the need to: after all, the genus concept significantly pre-dates Linnaeus.

With the full recognition that what I'm about to say has significant practical problems, I think that the (largely unconscious) underlying concept that most taxonomists have used for delimiting genera is that a genus is the smallest group of organisms that a non-expert could distinguish with a reasonable minimum of training. In this case, it makes sense that out genera have narrowed significantly since Linnaeus, because in many cases improved equipment and techniques have made studying many organisms (especially invertebrates and such) much easier for us than for Linnaeus.

In my next paper to be submitted, I have to compose a rant about the horribleness (and uselessness) of genera. The last two paragraphs suggest that I have quite a way to go.

That idea works for many fields as a very rough rule of thumb, but it's pretty easy to think of exceptions. In Mesozoic dinosaurs, for example, the majority of genera are monotypic (and even the large ones, like Psittacosaurus, are still tiny compared to those in other fields), even when species in different genera are very similar to each other. (Ornithomimids are a very good example of this.) On the other side of the coin, lions and tigers are very easy for anyone to distinguish, even without training, but they are universally considered members of the same genus.

As for my last two paragraphs--the penultimate one really only makes the case that the names are useful on their own. It really doesn't matter whether they're tied to the rank of genus or not. And the last paragraph shows how hard it would be to change to another system from the current one--but that doesn't mean the current system is very good.

I was referring to my last two paragraphs, not yours. Just attempting a bit of self-deprecation ;-).

Funnily enough, the lion/tiger example was exactly one that came to mind as an exception. I was wondering if it was due to vertebrate workers coming to regard osteology rather than overall appearance as the marker for distinctiveness. Interestingly, my impression when looking at works from the 1800s is that during that time most vertebrate workers used generally smaller genera than they do now.

(FWIW, when I was a young child for a brief time I thought lions and tigers were the same kind, with tigers the females and lions the males. Seeing an illustration of a lioness dispelled that.)

You think there's been a move toward relumping in vertebrate genera? Not sure if I see that as a general trend (and it's definitely not true for Mesozoic dinosaurs, where splitting is at an all-time high). Although, come to think of it, it is true of bears, with Thalarctos being long unused and Euarctos, Selenarctos, Helarctos and Melursus often being lumped into Ursus.